Although reverse breakdown of a diode is a departure from its rectifying action, practical use can be made of this effect. If a diode is supplied with reverse current from a current source with a sufficiently high voltage capability (>|BV|), the diode voltage is substantially constant over a wide range of current.

The diode, now used as a breakdown (or Zener) diode, has wide application in providing stabilized voltages ranging from 2.7 V to 200 V or more.

A breakdown diode is characterized by its nominal breakdown voltage and the reciprocal of the reverse characteristic in the reverse region, the dynamic slope resistance (rz).

An ideal breakdown diode has a well specified breakdown voltage and zero slope resistance giving a constant reverse voltage (in breakdown) indepen dent of temperature and reverse current.

In practice, however, the breakdown characteristic is curved in the low reverse current region and the reverse current supplied must be of sufficient magnitude to ensure that the breakdown diode operates beyond the knee of the characteristic in a region of low slope resistance.

Further, even beyond the knee, slope resistance varies with reverse current and depends on the nominal breakdown voltage and temperature. Manufacturers’ data should be consulted for accurate figures. In general, rz is a minimum for devices with a |BV| of approximately 6 V and operated at high reverse currents.

At lower currents and for both higher and lower values of |BV|, rz increases. The temperature coefficient of breakdown voltage depends on both the nominal breakdown voltage and on the reverse current.

Below approximately 5 V the temperature coefficient is negative and above is positive. This is because different breakdown mechanisms occur for low and high breakdown voltages.

At approximately 5 V both mechanisms are present and produce a zero temperature coefficient. The device rating which is important for a breakdown diode is the power dissipation, the product of reverse current and breakdown voltage.

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